The present invention relates to radiographic grids for use in an x-ray apparatus, especially for use in an x-ray mammography apparatus.
It has been well known since the early days of radiography that secondary or scattered x-rays reduce the contrast of the x-ray image. The low difference in x-ray absorption characteristics between cancerous and noncancerous tissue has made mammography particularly susceptible to imaging problems caused by scattered radiation. A conventional Bucky grid consisting of a series of lead foil strips separated by strips of x-ray transparent spacers helps remove scattered radiation from radiographic fields.
The thin strips of x-ray radiation absorbing material are called lamellae and are substantially aligned with the incident course of the radiation from the x-ray source, with the x-rays being transmitted through the gaps between the lamellae. The grid is positioned between the object and image receptor to reduce scatter radiation thereby improving image contrast on the film. The degree of alignment required of the lamellae is a function of the ratio of the height of the lamellae to the width of the gaps between the lamellae. Radiation not aligned with radiation from the source is blocked by the grid from reaching the film.
Bucky grids used in mammography are either stationary flat grids or reciprocating flat grids having an interspace material between adjacent lamellae. U.S. Pat. No. 4,901,335 to Ferlic et al. teaches a reciprocating grid having at least a 90% open area at all positions of its travel to transmission of directly incident x-ray radiation (i.e. radiation perpendicular to the tangent of the direction of travel of the grid at the point of incidence). The x-ray transmitting slots are formed between x-ray absorbing lamellae, which extend radially in directions from an axis through the x-ray source. The spacing between the lamellae of the grid is air to reduce x-ray absorption. At least a 7:1 grid ratio between lamellae height to open air gap width, compared to grid ratios between 2:1 and 5:1 for grids having interspace material between the lamellae, is used to minimize transmission of a scattered x-ray radiation through the grid, thereby enhancing image quality.
The large height to gap ratio with air spaced gaps between the lamellae is obtainable in part by use of radially aligned lamellae, which substantially eliminates grid focus problems. Thus, maximum aligned transmission is achieved at all points of the grids travel. A low density of lamellae in the grid, typically on the order of three to six lamellae per centimeter, reduces the need for exceptionally close tolerances in manufacturing of the lamellae and permits the use of lamellae of sufficient width so as to be substantially self supported between opposed side walls of the grid housing and between a top and a bottom cover sheet so that the interspace material is not necessary for supporting the lamellae.
Although the lamellae are supported independently within the grid housing it has been difficult to maintain the lengthwise straightness of the lamellae as they are assembled. In U.S. Pat. No. 4,901,335 Ferlic et al teaches that lamellae are individually positioned and aligned with respect to each other in a grid housing and then the cover sheet is substantially covered with an adhesive and pressed down onto the edges of the lamellae. This process is extremely time consuming and subject to the inherent tolerances of the technician assembling the grid. It typically on the order of several hours and even days to assemble a grid as described in the '335 patent. In addition, if the lamellae are not initially positioned straight and parallel to each other or if the lamellae become dislodged from the adhesive and warp, then there is a reduction in the transmission of the primary x-ray radiation and artifacts occur due to the misalignment.
Another method for attaching lamellae within grid housing has been to provide carbon fiber plates having a thickness of 1 mm on the top and bottom of individual tantalum lamellae. The carbon fiber plates have grooves in them to provide the proper spacing between the lamellae and then the lamellae are glued into the individual slots one at a time using an adhesive. This method, like the method above, is extremely time consuming and results in artifacts if the lamellae become dislodged or warped.